I have a similar boat, a 26' flybridge cruiser, weighs 7500# with a 260
hp Mercruiser 5.7. I have the stuff Harry refers to, a Standard Horizon
fuel flow meter and a GPS. I recently returned from a trip up and down
the Inside Passage to Alaska, so fuel consumption was a big deal.

I get 1.3 nautical miles/gallon on plane, cruising at 20 knots. I get 2
nautical miles/gallon at my "hull speed" which I figure is about 6
knots. This is actually the speed I almost always travel at, as like to
go slow, it saves fuel and the motor sounds the happiest.

Although if I had gone this speed on that trip I doubt if I would have
made it to Prince Rupert by now.